Thanks chaps.
The canvas is muslin cloth (previously used to filter honey as it happens) draped over a plasticard structure to give the frames. Then slapped with loads of PVA glue. The glue soaks into the muslin and stiffens it nicely. I'll probably cut some of the strucutre away inside and backfill it with something (possibly miliput or DAS clay) to make it lass bulky and more robust for casting.
The steering wheel will be a pain, they always are... I think I'll laser cut a frame then make the rim with wire.